A wind turbine is an energy converting device that converts kinetic energy in the wind into electrical energy for use by customers connected to a utility power grid. This type of energy conversion typically involves using wind to turn turbine blades that, in turn, rotate the rotor of an alternating current (AC) electrical generator, either directly or through a gearbox.
The primary electrical output of an AC generator is from its stator. The output from the stator can be directly connected to the grid or pass through a power converter. One common generator of prior art systems is the doubly-fed induction generator (DFIG) where the output from the stator is controlled by the current in its rotor. The stator in such a system can be directly connected to the grid because the stator voltage and frequency, being controlled by the rotor, can be forced to match the grid voltage and frequency.
A non-DFIG generator such as a synchronous generator or cage induction generator machine can also be used as the electrical generator in a wind turbine system that provides controlled real and reactive power. When these machine types are used in a variable speed configuration a full converter is utilized between the stator output and a utility grid since the output frequency of the generator is not controlled. A full converter rectifies the AC output of the stator to DC, and then inverts the DC back to AC at levels that match the grid voltage and frequency.
The electrical power available from a wind turbine and supplied to a utility grid is a function of the speed of the wind, the characteristics of the wind turbine rotor, efficiency of the wind turbine and associated equipment, losses in the grid and the characteristics of the distribution system and loads connected thereto. Because the wind speed and loads fluctuate, voltage levels in the grid can vary. Likewise since most electric power transmission components have a significant reactive component, voltages in the grid are also a function of the reactive characteristics of loads and components connected to the grid.
To prevent damage to equipment, grid voltage must be held within certain tolerances and when these limits are exceeded, action must be taken. For variations on the order of nominal voltages +/−5% or so, suppliers or absorbers of variable amounts of reactive power are used to compensate for voltage changes due to the reactive nature of the grid.
When a shorting type grid fault occurs, grid voltages can fall far below normal level, which could potentially damage power generating equipment due to, among other things, excess currents and mechanical stresses. As is well recognized, power equals voltage times current. Therefore, if there is a fall in voltage and the power is not reduced, the current will rise dramatically.
To protect against damage, if the fall in voltage exists for a significant period of time, circuit breakers or fuse like devices isolate that portion of the grid containing the fault from the power source. The isolation of the portion of the grid containing the fault is referred to as “clearing.”
Wind turbines and/or wind parks using constant power controllers (both real and reactive power) are particularly susceptible to damage by excess currents. If the power control loop can not respond quickly enough, output current will spike up to maintain a fixed power level and compensate for a falling voltage. To protect against high currents, many prior art systems limit output current during a fault current to a maximum preset value, thus minimizing potential damage due to high current.
The present invention uses a control concept commonly described as a constant current or controlled current source for protection. As used in this specification, the terms “constant current” or “controlled current” are to be interpreted to mean that the constant or controlled current is related to a command or reference by a substantially fixed proportionality factor and substantially independent of the voltage into which the current is fed.
The power output of wind turbine operating as a constant or controlled current source varies in direct proportion to the grid voltage. Therefore, voltage during a fault goes through two phases of varying output power. When the voltage drops in response to the fault, the output power is at a minimum, having gone through a substantially steep transition from full power to a much lower level. After the fault is cleared, the grid voltage rises, which demands an increasing amount of output power from the wind turbine(s) until the power is restored to pre-fault levels.
In the past it was more common to protect the turbines affected by the fault by disconnecting them from the grid. However, as the number of wind turbines being used to generate electricity grew, increasing their relative contribution to the overall power of the grid, emerging practice is to require that wind turbines stay connected to help support the grid both during a fault and as the grid recovers from the fault, that is provide, as needed, either reactive or real current.
The support requirement generally relates to counteracting the effect of a fault by trying to raise system voltages and to minimize the amount of time required to place a wind turbine back on line generating power. Moreover, although it seems counterintuitive, it is advantageous for affected wind turbines to continue to provide output current at substantially the same magnitude as that which was present prior to the fault and not reduce it. A normative output current is better able to actuate the protective devices and therefore potentially shorten the time to isolate a fault.
An additional motivation for keeping a wind turbine connected and providing power during a fault is to reduce the time needed to get a disconnected turbine back on line after the fault is cleared. If the wind turbine rotor could be kept running at or near the pre-fault speed, the time to get fully back on line can be greatly shortened. Conversely, if the load is reduced, as it is in a fault condition, the blades of the turbine would be caused to accelerate rapidly, and unless some steps are taken to address the problem, damage will occur.
Grid codes around the world require different behavior during a low voltage grid fault. Some grid codes requires full reactive current and as much as possible active current during the grid disturbance. Other grid codes prioritize the active current. Although it would be desirable to be able to maximize real and reactive current at the same time, component heating, whether it is the rotor of a DFIG or the current carrying elements of a partial or full converter, is a function of both the real and reactive components of the current being carried. Therefore if it is desired to maximize real current, then the reactive current component must be minimized. Likewise if reactive current were to be maximized, then the real current component of the total must be minimized.
The present invention is directed to solving the aforementioned problem of being able to maximize both the reactive and real current components available from a wind turbine or grouping of wind turbines, by providing a separate reactive power supply to handle reactive current requirements during a fault and requiring the wind turbines themselves to maximize real power.
As is known to those skilled in the art, a variable speed wind turbine extracts the maximum it can from the wind when the blade tip speed to wind speed ratio is a constant at or near the particular design value for the particular wind turbine design. However, because of generator speed restrictions, it is not possible to operate a wind turbine at its optimum blade tip-wind speed ratio over the whole wind speed range. Stated differently, as the wind speed increases, the rotational speed of the generator increases and brings it closer to the upper speed limit of the rotor and generator.
In the mid- to upper-speed range, which is the range at where the wind turbine is most efficient, the blade tip to wind speed ratio is held constant by balancing the power output from the system to that available from the wind. That is, the power commanded by the wind turbine is derived from knowledge of the wind speed and is set to that value.
At higher wind speeds the wind turbine cannot be allowed to operate at its optimum blade tip to wind speed ratio, known as the tip speed ratio (typically in the range of about 6 to 10) because to keep the ratio constant would require a rotational speed for the generator that would exceed its limits. Therefore when wind speed increases to its nominal speed, and further increases could drive the generator into an unsafe speed range, a generator speed reference is clamped to the nominal speed point. If wind speed increases further, the commanded power output is limited to a fixed value and blade pitch is varied to keep the power taken from the wind equal to the power necessary to keep the wind turbine rotor, and hence the generator, at the generator's nominal speed. Blade pitch control is used because, even though there is more power available to be extracted from the wind at high wind speeds, the wind turbine rotor is made less efficient at extracting the extra power.
In the case of higher wind speed (as in the case of the lower speed range as well), there must be a balance between power captured by a rotor system and the power outputted from the turbine plus losses in the various wind turbine systems. However a difference between operation in these ranges is that in the optimum speed range, the blade is set to extract as much power is available, while in the higher speed range the blade pitch is set to take just enough power to meet the value commanded for full power which is less than what is actually available in the wind.
When either balance is disturbed, as in the case of a sudden low voltage fault, prior art systems have often used a rapid blade pitch change to a no power position followed by shutdown to prevent equipment damage due to over currents and overspeed conditions.
In general, the power captured by a wind turbine rotor blade system is derived from:
                                          P            el                    =                                    ηρ              2                        ⁢            π            ⁢                                                  ⁢                          R              2                        ⁢                          c              p                        ⁢                          v              3                                      ,                            [                  Eq          .                                          ⁢          1                ]            where η is an efficiency factor, dependent upon the efficiency of the generator, gearbox, etc. and ρ is air density, which is about 1.225 kg/m3 at sea level; R is rotor radius in meters, cp is the fraction of power extracted from the wind, and v is the wind velocity in meters/second.
A prior art set of cp curves as a function of blade pitch angles and blade tip to wind speed ratio (lambda) are shown in FIG. 1. As illustrated in FIG. 1, each different blade pitch angle curve has a different blade tip to wind speed ratio at which cp is a maximum. A plot of the blade pitch angle versus blade tip to wind speed ratio at each of the maximum cp points therefore yields the blade pitch angle that will allow extraction of maximum available power at each lambda. As one skilled in the art would recognize from the example in FIG. 1, the maximum power in the wind will be extracted if the tip to wind speed ratio is in the ratio of about 6 to 10.
For a given wind turbine design, Eq. 1 calculates the actual electrical output of a wind turbine when both cp, which is a function of blade pitch angle, and v are known.
Likewise, the same constituents of Eq. 1 can be used for calculating cp rather than Pel, as follows:
                                          c            p                    =                                    2              ⁢                              P                el                                                    ηρπ              ⁢                                                          ⁢                              R                2                            ⁢                              v                3                                                    ,                            [                  Eq          .                                          ⁢          2                ]            Eq. 2 therefore gives the power coefficient value needed to extract from a wind at velocity v, an electrical value of Pel, as a function of lambda.
The present invention utilizes the information in FIG. 1, Eq. 1 and Eq. 2 to determine a blade pitch angle that will extract from the wind the power imposed on a wind turbine during and after a fault and therefore hold generator rotor speed substantially constant during and after a fault.
One additional benefit of the present invention is that it improves recovery characteristics by providing controlled current values while tolerating the varying voltages that result from a fault. This compares favorably to the prior art systems, where the power delivered to the load during a fault is a transient function of changing voltage and current.